In order to compare collectors, test institutions usually estimate efficiency curves based on measurements of collector performance. This curve shows a variety of temperature differences between collectorTC and ambient air TA, with E the available irradiance in W/m2. The commonly used empirical equation for the collector efficiency n is:
* n = n0 – (a1·(TC-TA)+a2·(TC-TA)²)/E
Graph of efficiency and average temperature difference at constant irradiance level of 1000 W/m2, for typical solar collectors.